The purpose of this project is to investigate age-related changes in thermoregulation and to examine the physiological mechanisms underlying these changes. We have demonstrated that aged mice have diminishing cold tolerance and are not able to adapt to repeated cold exposure. The cause of these age-related aberrations in thermoregulation appears to be, in part, a reduction in metabolic heat production and, in part, a reduction in heat conservation mechanisms. During the life span, the relative contributions of heat production and heat conservation mechanisms to cold tolerance change. The results from a number of experiments suggest that the mechanism responsible for diminished metabolic heat production in response to cold is an age-related change in brown adipose tissue. Collaborative studies with scientists at the Tokyo Metropolitan Institute of Gerontology suggest that these changes are associated with changes in vascular function.